1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by an electrophotographic method.
2. Description of the Related Art
When multi-level image data is printed as by a digital copier or laser printer utilizing electrophotography, use is made of tone representation that relies upon a density pattern method.
FIG. 1 illustrates the smallest unit (pixel) for representing density by the density pattern method that expresses tone of one pixel in accordance with number of filled-in dots within an N×M dot pattern. In a case where N or M is 1, the result is a one-dimensional block. If N and M are 2 or greater, then the result is a two-dimensional block. Take a two-dimensional block as an example. With a binary printer having a resolution of 600 dpi, a total of 17 (16+1) tones can be expressed by adopting 4×4 dots as the smallest unit (block) and changing the number of black dots inside the block in accordance with the density of the pixel.
FIG. 2 is a diagram illustrating a typical one-dimensional block [1×N dots (N=16)]. It should be noted that the numerals in FIG. 2 indicate the order in which black dots are assigned in accordance with tone by the density pattern method. For example, the ninth dot is only formed when density of image data is “1”, and the ninth dot and the eighth dot are formed when density of image data is “2”, and the like (see FIG. 6).
FIG. 3 is a diagram illustrating the configuration of an image forming system according to the prior art.
As shown in FIG. 3, a printer 401 is internally provided with a controller 410, an image processor 420 and an exposure unit 430. If, upon receiving image data (inclusive of a photographic image or character image, etc.) from a host computer 400, the image data represents a character image, then the controller 410 converts the image data to a prescribed bitmap signal (e.g., white is made “00h” and black is made “1Fh”), where h represents a hexadecimal number. If the image data represents a photographic image, on the other hand, then the controller converts the image data to code signal (from white “00h” to “1Fh” (black)) the numerical value of which increases as density increases, and sends the code to the image processor 420. The image processor 420 subjects this image data to various image processing and sends the resultant signal to the exposure unit 430. The exposure unit 430 modulates a laser beam in accordance with the output signal from the image processor 420, exposes a photosensitive member and prints the image through the processes of development, transfer and fixing.
It has been pointed out that with the density pattern method, tone reproduction of halftones utilizing the ability of a printer fully cannot be achieved for the following reasons:
(1) the graphed points in the density characteristic of the photosensitive member of the exposure unit 430 are not evenly spaced (linear) with respect to the image data;
(2) the graphed points in the development characteristic are not evenly spaced (linear) with respect to the image data; and
(3) the graphed points in the transfer characteristic are not evenly spaced (linear) with respect to the image data.
Further, in each of the steps relating to image formation, the correlation between the image data and the tone of the image formed is extremely complex, and there are instances where the tonality of the image finally obtained through each step differs from the tonality of the input image data.
More specifically, the tonality of the image formed varies depending upon the type of photosensitive member (material and film thickness, etc.); the development scheme (one-component or two-component, contact or contactless); the bias applied in charging, developing and transfer; the amount of developer electrically charged; and the type of transfer member, etc. In addition, the tone characteristic may vary greatly depending upon the state of the apparatus when used initially and after use over a long period of time, or depending upon use in environments of different temperature and humidity.
In order to solve this problem, Japanese Patent Application Laid-Open No. 2000-350025 describes that excellent tone reproduction of halftones is performed by creating density patterns greater than a number of density patterns determined by a prescribed number of dots and changing over the density patterns to be used. Further, there are also cases where filled-dot patterns among these density patterns are discretely configured and then tone reproduction of halftones is carried out using the discretely configured patterns. The fact that an image forming apparatus using the electrophotographic method has the characteristics set forth below can be mentioned as a reason for this.
(1) In a case where adjacent dots have been formed, if the laser spot diameter becomes too large, then a portion in which the dots overlap is produced.
(2) The characteristic (relationship between exposure energy and drum surface potential) of amount of exposure vs. electric potential of a photosensitive drum is not linear.
(3) The development characteristic (reflective density vs. development contrast) is not linear.
Owing to these characterizing features, the relationship between the density of input data and optical density of the output image loses its linearity in dependence upon the contiguity of filled-in dots. Consequently, the relationship between halftone data and the tone of an output image varies owing to a difference in the order (image signal density pattern) in which dots are filled in (made black) in accordance with a tone of the kind mentioned above. Conceivable as a method of dealing with this is to provide a plurality of types of density patterns for reproducing halftones and change over the density pattern used in conformity with the image formation characteristic of the apparatus.
In a case where a plurality of types of density patterns for reproducing halftones are provided, the number of density patterns that must be set in advance becomes enormous when individual differences among apparatuses and changes in environment are taken into account. As a consequence, the storage capacity for storing the density patterns increases and this invites higher cost. Further, the processing for detecting the density of a formed image and selecting the optimum density pattern from among such an enormous quantity of density patterns based on the detected density is complicated and difficult to implement.